1. Field of the Invention
This invention relates to a perovskite oxide; an inorganic composition or an inorganic molded body or inorganic film containing therein the perovskite oxide; a ferroelectric element using a perovskite oxide; and a piezoelectric actuator or a liquid discharge system using a ferroelectric element.
2. Description of the Related Art
There has been used in various applications such as the piezoelectric actuator for an inkjet recording head a piezoelectric actuator comprising piezoelectric film having piezoelectricity where it is expanded and contracted according to the intensity of an applied electric field and an electrode for applying an electric field to the piezoelectric film. As a piezoelectric material, a perovskite oxide having a ferroelectricity such as zircon titanate (PZT) has been wide used.
While electronic devices have been miniaturized, light-weighted and provided with various functions, there is a tendency to make the piezoelectric element mounted thereon miniaturized, light-weighted and provided with various functions. For example, in the inkjet recording head, to increase the density of the piezoelectric elements has been investigated in order to improve the image quality and to reduce the thickness of each piezoelectric element in response thereto has been investigated. Accordingly, piezoelectric film exhibiting a high piezoelectricity even its thickness is small becomes necessary.
That the piezoelectric film exhibits a high piezoelectricity in the vicinity of a phase border is known and, for instance, in PZT, Zr and Ti are used in a molar ratio of 52/48 equal to that in the vicinity of a phase border. However, in the thin type piezoelectric film applications, a higher performance piezoelectric material becomes necessary.
As a means for improving the piezoelectricity, there has been proposed to dope the B site of PZT with an V or VI group element. For example, in U.S. Pat. No. 7,196,457, there is disclosed piezoelectric film having a composition represented by the following formula.A1−bB1−aXaO3 wherein A represents at least one kind of element including Pb, B represents Zr and/or Ti, X represents at least one kind of element of V, Nb, Ta, Cr, Mo and W, 0.05≦a≦0.3, and 0.25≦b≦0.15.
In U.S. Pat. No. 7,196,457, there is disclosed that Pb in the A site is apt to evaporate to generate A site defect since the vapor pressure of Pb is high in PZT and when Pb is dislocated from the A site, oxygen loss is simultaneously generated on the basis of the law of charge neutrality (this defect is referred to as “Schottoky defect”), and when oxygen loss is generated, the band gap of PZT lowers and the insulation lowers whereby current leak is apt to be generated. Further, there is disclosed that, in the piezoelectric film of U.S. Pat. No. 7,196,457, when B site is doped with +5- or +6-valence element, oxygen loss is not generated even if Pb loss is generated, whereby charge neutrality can be held and current leak can be suppressed.
In the above formula, the number of moles of the A site element is represented by “1-b”, and “b” represents Pb loss. That is, the invention disclosed in U.S. Pat. No. 7,196,457, that Pb loss is generated is assumed and oxygen loss is suppressed from being generated simultaneously with generation of Pb loss by doping an element having a valence larger than +4 to the B site which is originally +4 in its valence.
In U.S. Pat. No. 7,196,457, piezoelectric film 200 pm/V (=−200 pC/N) in piezoelectric constant (d31) is obtained. The doping amount of X which is at least one kind of V group and/or VI group elements is 5 to 30 moles.
In “Effect of Nb doping on the microstructural and electrical properties of the PZT ceramics”, M. Pereira et al., J. Eur. Ceram. Soc. Vol. 21, Issues 10-11, pp. 1353-1356, 2001, there is disclosed that up to about 7 mol % of Nb can be doped to PZT by solid solution in bulk ceramics, and when Nb is doped in about 2 mol %, the piezoelectric constant is maximized. Generally, the limit of solid solution can be obtained by the relation with the ionic radius or the like.
Inventors of U.S. Pat. No. 7,196,457 has described in the academic society that since the crystallization temperature is increased when doped with Nb, it is necessary to increase the firing temperature and when the firing temperature is increased, there arises a problem that Pb is dislocated and accordingly, the limit of solid solution is as disclosed in “Effect of Nb doping on the microstructural and electrical properties of the PZT ceramics”, M. Pereira et al., J. Eur. Ceram. Soc. Vol. 21, Issues 10-11, pp. 1353-1356, 2001 in the normal method irrespective of whether the PZT is of bulk or of film. The inventors insist that the firing temperature can be lowered by adding Si as a firing assistant, whereby dislocation of Pb is suppressed and Nb can be doped in a high concentration.
It has been described above that generation of Pb loss is assumed and oxygen loss is suppressed from being generated simultaneously with generation of Pb loss by doping an element having a valence larger than +4 to the B site which is originally +4 in its valence, in the invention disclosed in U.S. Pat. No. 7,196,457. In other words, in the invention disclosed in U.S. Pat. No. 7,196,457, cation defect is necessarily generated in the A site on the basis of the law of charge neutrality since the B site is doped with an element having a valence larger than +4. The cation defect is not preferred in that it can lead to deterioration of ferroelectric performance (piezoelectric performance) and the like.